Conductive paste, method, electrode and solar cell

ABSTRACT

A conductive paste for forming a conductive track or coating on a substrate, the paste comprising a solids portion dispersed in an organic vehicle, the solids portion comprising electrically conductive material and an inorganic particle mixture; wherein the inorganic particle mixture comprises particles of glass frit and substantially crystalline particles of one or more metal compounds; and wherein the glass frit comprises at least 90 mol % Te O 2 .

FIELD OF THE INVENTION

The present invention relates to conductive pastes which areparticularly suitable for use in solar cells and methods for makingthose pastes, to a method of manufacturing a conductive track or coatingon a surface e.g. of a solar cell, and to an electrode and to a solarcell having such an electrode formed on a surface thereof.

BACKGROUND OF THE INVENTION

Conductive (e.g. silver-containing) pastes are routinely used in thepreparation of conductive tracks for solar cells, such as silicon solarcells. The pastes typically comprise conductive (e.g. silver) powder,glass frit, and sometimes one or more additional additives, alldispersed in an organic vehicle. In the manufacture of solar cells, sucha paste is typically applied to a semi-conductor substrate (e.g. awafer) via screen-printing and is subsequently fired (i.e. subjected toheat treatment). The glass frit has several roles. During firing, itbecomes a molten phase and so acts to bond the conductive track to thesemiconductor wafer. However, the glass frit is also important inetching away the anti-reflective or passivation layer (usually siliconnitride) provided on the surface of the semiconductor wafer, to permitdirect contact between the conductive track and the semiconductor. Theglass frit is typically also important in forming an ohmic contact withthe semiconductor wafer.

Glass frits which find use in conductive paste applications oftencontain lead. However, the use of lead is undesirable due toenvironmental and toxicity concerns.

The quality of the contact between the conductive track and thesemiconductor wafer is instrumental in determining the efficiency of thefinal solar cell. The best glass frits need to be optimised to flow atthe correct temperature, and to provide the correct degree of etching ofthe antireflective layer. If too little etching is provided, then therewill be insufficient contact between the semiconductor wafer and theconductive track, resulting in a high contact resistance. Conversely,excessive etching may lead to deposition of large islands of silver inthe semiconductor, disrupting its p-n junction and thereby reducing itsability to convert solar energy into electrical energy.

Much recent attention has focussed on improving the glass frit materialsincluded in conductive pastes for photovoltaic cells, to provide a goodbalance of properties. Alternatives to glass frits have also beenproposed, such as mixtures of crystalline oxides. Conductive pastescomprising conductive powder, glass frit, and sometimes one or moreadditional additives, all dispersed in an organic vehicle, are also usedto form conductive tracks or conductive coatings in a range of otherelectronics applications, including passive electronic components, e.g.in terminal electrodes for zinc oxide varistor components, terminationsfor MLCC (multi-layer ceramic capacitors), electrodes on TCO(transparent conductive oxide) coated glass substrate, conductive layerson NTC (negative temperature coefficient) thermistors, metallization offunctional piezoceramics; and automotive applications including antennaeand heatable mirrors, windscreens and backlites.

SUMMARY OF THE INVENTION

There remains a need for compositions suitable for use in conductivepastes for solar cells which provide an excellent (lowered) contactresistance without negatively influencing the p-n junction of a solarcell, and which flow at a suitable temperature for firing the conductivepaste during manufacture of a solar cell. Furthermore, there remains aneed for conductive pastes which do not contain toxic components, suchas lead, and which the manufacturing, recycling and disposal thereof hasa reduced impact on the environment.

The present inventors have surprisingly found that the use of glass fritcombined with substantially crystalline particles of one or more metalcompounds may provide a conductive paste with superior properties.

Accordingly, a first aspect of the present invention provides aconductive paste for forming a conductive track or coating on asubstrate, the paste comprising a solids portion dispersed in an organicvehicle, the solids portion comprising electrically conductive materialand an inorganic particle mixture; wherein the inorganic particlemixture comprises particles of glass frit and substantially crystallineparticles of one or more metal compounds; and wherein the glass fritcomprises at least 90 mol % TeO₂.

Conductive pastes according to the present invention, which comprise acombination of glass frit and substantially crystalline particles of oneor more metal compounds, offer a number of advantages over otherconductive pastes for solar cell applications. When used in solar cellapplications, conductive pastes of the present invention have been foundto provide solar cells with improved (i.e. lower) series resistance andhence improved conductivity.

According to a second aspect of the present invention, there is provideda method of preparing a conductive paste according to the first aspect,comprising mixing an organic vehicle and the components of the solidsportion, in any order.

According to a third aspect of the present invention, there is provideda method for the manufacture of an electrode of a solar cell, the methodcomprising applying a conductive paste as defined in the first aspect toa semiconductor substrate, and firing the applied conductive paste.

According to a fourth aspect of the present invention, there is providedan electrode for a solar cell, the electrode comprising a conductivetrack on a semiconductor substrate, wherein the conductive track isobtained or obtainable by firing a paste as defined in the first aspecton the semiconductor substrate.

According to a fifth aspect of the present invention, there is provideda solar cell comprising an electrode as defined in the fourth aspect.

According to a sixth aspect of the present invention, there is providedthe use of a conductive paste as defined in the first aspect in themanufacture of an electrode of a solar cell.

According to a seventh aspect, there is provided the use of an inorganicparticle mixture in a conductive paste to improve the series resistanceof a solar cell, wherein the inorganic particle mixture comprisesparticles of glass frit and substantially crystalline particles of oneor more metal compounds and wherein the glass frit comprises at least 90mol % TeO₂.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows an example of a firing curve for a solar cell prepared inthe Examples.

DETAILED DESCRIPTION

Preferred and/or optional features of the invention will now be set out.Any aspect of the invention may be combined with any other aspect of theinvention unless the context demands otherwise. Any of the preferredand/or optional features of any aspect may be combined, either singly orin combination, with any aspect of the invention unless the contextdemands otherwise.

For example, the discussion of the inorganic particle mixture content,raw materials and particle size distribution is applicable to theaspects of the invention relating to pastes and methods equally.

Where ranges are specified herein it is intended that each endpoint ofthe range is independent. Accordingly, it is expressly contemplated thateach recited upper endpoint of a range is independently combinable witheach recited lower endpoint, and vice versa.

Solids Portion

Conductive pastes of the present invention include an organic vehicleand a solids portion. The solids portion includes an electricallyconductive material and an inorganic particle mixture. The inorganicparticle mixture comprises particles of glass frit and substantiallycrystalline particles of one or more metal compounds. Variousembodiments of the solids portion will be discussed in greater detailbelow, as will various methods of utilising them to form a conductivepaste.

The solids portion may constitute at least 80 wt %, at least 85 wt %, atleast 87 wt % or at least 90 wt % of the conductive paste. The solidsportion may constitute 98 wt % or less, 95 wt % or less, or 91 wt % orless of the conductive paste. In some embodiments, the conductive pastemay comprise ≥80 to ≤98 wt % of the solids portion.

Conductive Material

In some embodiments, the solids portion includes ≥80 to ≤99.9 wt % ofelectrically conductive material (with respect to total weight of thesolids portion). In some embodiments, the electrically conductivematerial is a metallic silver powder. However, the invention is notparticularly limited to metallic silver powders and powders of otherelectrically conductive materials are contemplated.

Inorganic Particle Mixture

The solids portion of the conductive paste of the present invention mayinclude ≥0.1 to ≤15 wt % of the inorganic particle mixture (with respectto total weight of the solids portion). The solids portion of theconductive paste may include at least 0.5 wt % or at least 1 wt % of theinorganic particle mixture. The solids portion of the conductive pastemay include 10 wt % or less, 7 wt % or less or 5 wt % or less of theinorganic particle mixture.

The inorganic particle mixture comprises particles of glass frit andsubstantially crystalline particles of one or more metal compounds,wherein the glass frit comprises at least 90 mol % TeO₂. Detaileddescriptions of each component of the inorganic particle mixture are setout hereinafter.

It will be understood by the skilled reader that a glass material is notsynonymous with an amorphous material, or even an amorphous regionwithin a crystalline material. A glass material exhibits a softeningpoint and does not exhibit a melting point. A glass material exhibits aglass transition. While glasses may include some crystalline domains(they may not be entirely amorphous) these are different from thediscrete substantially crystalline particles of one or more metalcompounds required by the present invention.

The term “substantially crystalline” means crystalline material whichhas long-range structural order of atoms through the material. Such amaterial does not exhibit a glass transition. This contrasts with, forexample, amorphous or glassy materials. Substantially crystallinematerials exhibit a melting point.

The person skilled in the art will be able to select suitable methods todetermine whether a material is crystalline or amorphous. For example,the person skilled in the art may use X-ray diffraction (XRD) methods.An amorphous or glass material will not produce distinct peaks in an XRDpattern. On the contrary an amorphous or glass material will producebroad signals in an XRD pattern. A substantially crystalline materialwill give rise to multiple distinct peaks in an XRD pattern.

In some embodiments, the inclusion of the inorganic particle mixture inthe paste may provide improved contact resistance in solar cellsprepared using the conductive paste. In some embodiments, the use of aconductive paste comprising the inorganic particle mixture in thepreparation of a solar cell may provide a solar cell having increasedefficiency. In particular, it has been surprisingly found that the useof the conductive paste of the present invention in the preparation of asolar cell may provide a solar cell having increased efficiency comparedto a solar cell prepared using a paste which comprises an inorganicparticle mixture having the same overall chemical composition butprovided as only glass frit or only as a mixture of crystalline metalcompounds.

The inorganic particle mixture may consist essentially of particles ofglass frit as described herein and particles of a substantiallycrystalline material as described herein.

The inorganic particle mixture may comprise particles of glass frit inan amount of at least 25 wt. %, for example, in an amount of at least 40wt %, at least 45 wt % or at least 50 wt % (with respect to total weightof the inorganic particle mixture). The inorganic particle mixture maycomprise particles of glass frit in an amount of 75 wt % or less, forexample, 70 wt % or less, 65 wt % or less, or 60 wt % or less. In someembodiments, the inorganic particle mixture may comprise 40 to ≤70 wt %of particles of glass frit.

The inorganic particle mixture may comprise substantially crystallineparticles in a total amount of at least 20 wt %, for example, in anamount of at least 25 wt. %, at least 30 wt. %, at least 35 wt. % or atleast 40 wt % (with respect to total weight of the inorganic particlemixture). The inorganic particle mixture may comprise substantiallycrystalline particles in a total amount of 75 wt % or less, for example,60 wt % or less, 55 wt % or less, or 50 wt % or less. In someembodiments, the inorganic particle mixture may comprise 35 to ≤55 wt %.

The glass frit of the inorganic particle mixture comprises at least 90mol % TeO₂. In some embodiments, the glass frit may comprise greaterthan 90 mol % TeO₂. The glass frit may comprise at least 91 mol % TeO₂,preferably at least 92 mol % TeO₂, more preferably at least 95 mol %TeO₂.

The glass frit compositions described herein are given as molepercentages, on an oxide basis. These mole percentages are with respectto the total molar composition of the glass frit. The mole percentagesare the percentages of the components used as starting materials inpreparation of the glass frit compositions, on an oxide basis. As theskilled person will understand, starting materials other than oxides maybe used in preparing the glass frits of the present invention. Where anon-oxide starting material is used to supply an oxide of a particularelement to the glass frit composition, an appropriate amount of startingmaterial is used to supply an equivalent molar quantity of the elementhad the oxide of that element been supplied at the recited mol %. Thisapproach to defining glass frit compositions is typical in the art. Asthe skilled person will readily understand, volatile species (such asoxygen) may be lost during the manufacturing process of the glass frit,and so the composition of the resulting glass frit may not correspondexactly to the weight percentages of starting materials, which are givenherein on an oxide basis. Analysis of a fired glass frit by a processknown to those skilled in the art, such as Inductively Coupled PlasmaEmission Spectroscopy (ICP-ES), can be used to calculate the startingcomponents of the glass frit composition in question.

The glass frit may contain oxides of additional elements. In someembodiments, the glass frit may comprise an alkali metal oxide, analkaline earth metal oxide, an oxide of cerium, an oxide of bismuth, ormixtures thereof. The alkali metal oxide may be Li₂O, Na₂O, K₂O, ormixtures thereof. In some embodiments, the glass frit contains Li₂O andNa₂O. The alkaline earth metal oxide may be BaO, CaO, MgO, or mixturesthereof. The oxide of cerium may be Ce₂O₃ or CeO₂. The oxide of bismuthmay be Bi₂O₃.

The glass frit may include Li₂O. The glass frit may include at least 0.1mol %, at least 1 mol %, or at least 3 mol % of Li₂O. The glass frit mayinclude 10 mol % or less, 9 mol % or less, or 8 mol % or less of Li₂O.For example, the glass frit may include ≥3 to ≤8 mol % of Li₂O.

The glass frit may include Na₂O. The glass frit may include at least 0.1mol %, at least 1 mol %, or at least 3 mol % of Na₂O. The glass frit mayinclude 10 mol % or less, 9 mol % or less or 8 mol % or less of Na₂O.For example, the glass frit may include ≥3 to ≤8 mol % of Na₂O.

The glass frit may include K₂O. The glass frit may include at least 0.1mol %, at least 1 mol %, or at least 3 mol % of K₂O. The glass frit mayinclude 10 mol % or less, 9 mol % or less, or 8 mol % or less of K₂O.For example, the glass frit may include ≥3 to ≤8 mol % of K₂O.

The glass frit may include BaO. The glass frit may include at least 0.1mol %, at least 1 mol %, at least 2 mol %, at least 4 mol % or at least6 mol % of BaO. The glass frit may include 10 mol % or less, 9 mol % orless, 8 mol % or less or 7 mol % or less of BaO. For example, the glassfrit may include ≥0.1 to ≤10 mol % of BaO.

The glass frit may include CaO. The glass frit may include at least 0.1mol %, at least 1.0 mol %, at least 2 mol %, at least 4 mol % or atleast 6 mol % of CaO. The glass frit may include 10 mol % or less, 9 mol% or less, 8 mol % or less or 7 mol % or less of CaO. For example, theglass frit may include ≥0.1 to ≤10 mol % of CaO.

The glass frit may include MgO. The glass frit may include at least 0.1mol %, at least 1 mol %, at least 2 mol %, at least 4 mol % or at least6 mol % of MgO. The glass frit may include 10 mol % or less, 9 mol % orless, 8 mol % or less or 7 mol % or less of MgO. For example, the glassfrit may include ≥0.1 to ≤10 mol % of MgO.

The glass frit may include Ce₂O₃. The glass frit may include at least0.1 mol %, at least 0.5 mol %, at least 1 mol %, at least 2 mol % or atleast 4 mol % of Ce₂O₃. The glass frit may include 10 mol % or less, 8mol % or less, 6 mol % or less or 5 mol % or less of Ce₂O₃. For example,the glass frit may include ≥0.1 to ≤10 mol % of Ce₂O₃.

The glass frit may include Bi₂O₃. The glass frit may include at least0.1 mol %, at least 0.5 mol %, at least 1 mol %, at least 2 mol % or atleast 4 mol % of Bi₂O₃. The glass frit may include 10 mol % or less, 8mol % or less, 6 mol % or less or 5 mol % or less of Bi₂O₃. For example,the glass frit may include ≥0.1 to ≤10 mol % of Bi₂O₃.

The glass frit may include further components, such as further oxidecomponents. The glass frit may include at least 0.1 mol %, at least 0.5mol %, at least 1 mol %, at least 2 mol % or at least 4 mol % in totalof further components. The glass frit may include 10 mol % or less, 8mol % or less, 6 mol % or less or 5 mol % or less in total of furthercomponents. The further components may be one or more selected from thegroup consisting of Ge₂, Zr₂, CuO, Al₂O₃, B₂O₃, WO₃, MO₃, ZnO, Al₂O₃,RuO₂, PdO, V₂O₅ and P₂O₅.

In some embodiments, the glass frit may be substantially free oflead-oxide and/or substantially free of silicon oxide.

As used herein, the term “substantially free of” in relation to glassfrit composition means that the glass frit has a total content of therecited component of less than or equal to 1 mol %. As will be readilyunderstood by the skilled person, during manufacture of glass fritparticles, the glass composition may be contaminated with low levels ofimpurities. For example, in a melt/quench glass forming process, suchimpurities may derive from refractory linings of vessels employed in themelting step. Thus, whilst a total absence of a particular component maybe desirable, in practice this may be difficult to achieve. As usedherein in relation to glass frit, the term “no intentionally added X”,where X is a particular component, means that no raw material wasemployed in the manufacture of the glass frit which was intended todeliver X to the final glass composition, and the presence of any lowlevels of X in the glass frit composition is due to contamination duringmanufacture.

In some embodiments, the glass frit may comprise less than 0.5 mol %,preferably less than 0.25 mol %, more preferably less than 0.05 mol %,most preferably less than 0.01 mol % PbO.

In some embodiments, the glass frit does not include any intentionallyadded lead.

In some embodiments, the glass frit may comprise less than 0.5 mol %,preferably less than 0.25 mol %, more preferably less than 0.05 mol %,most preferably less than 0.01 mol % SiO₂. In some embodiments, theglass frit does not include any intentionally added silicon.

Typically, the glass frit is prepared by mixing together the startingmaterials, melting them to form a molten glass mixture and thenquenching the molten mixture to form the glass frit. The process mayfurther comprise milling the frit to provide the desired particle size.

The skilled person is aware of alternative suitable methods forpreparing glass frit. Suitable alternative methods include, sol-gelprocesses and spray pyrolysis.

Whilst the glass frits according to the present invention are describedas comprising oxides of respective elements, the choice of startingmaterial used to prepare the frit may be any compound which decomposesto an oxide upon formation of a glass. For example, suitable startingmaterial compounds used to prepare glass frits may be oxides,carbonates, nitrates, hydrogen carbonates, oxalates, acetates and/orformates of said element. Some specific starting material compoundswhich may be used in forming the glass frit are Li₂O, Li₂CO₃, Bi₂O₃,Bi₅O(OH)(NO₃)₄, Ce₂O₃, CeO₂, Na₂O and Na₂CO₃.

The glass frit may be milled or ground to provide a desired particlesize and a desired particle morphology.

The inorganic particle mixture further comprises substantiallycrystalline particles of one or more metal compounds. Since theparticles are substantially crystalline, they do not exhibit a glasstransition.

For the avoidance of doubt, the term “metal”, used herein in relation tothe substantially crystalline particles of one or more metal compounds,includes metalloids, such as boron, silicon, germanium, arsenic,antimony and tellurium. Thus, the substantially crystalline particles ofone or more metal compounds may comprise a compound of tellurium, and/ora compound of boron, for example.

The particulate nature of the substantially crystalline particles of oneor more metal compounds means that discrete, separate or individualparticles of each of the one or more metal compounds are present in theinorganic particle mixture.

In some embodiments of the invention the inorganic particle mixturecomprises substantially crystalline particles of more than one metalcompound, in some embodiments two or more metal compounds, in someembodiments three or more, four or more, five or more or six or moredifferent metal compounds.

The substantially crystalline particles of one or more metal compoundsmay include a compound of lithium, sodium, potassium, barium, cerium,bismuth, tungsten, molybdenum, vanadium, calcium, magnesium, manganese,silver, boron, zinc, zirconium, tellurium, silicon, chromium or mixturesthereof. The one or more metal compounds may be selected from metaloxides, carbonates, nitrates, hydrogen carbonates, oxalates, acetatesand/or formates. Where the one or more metal compounds include non-oxidecompounds, preferably the non-oxide compounds are compounds which woulddecompose to oxides on firing. The person skilled in the art will beable to select other suitable metal compounds.

Where the substantially crystalline particles comprise two or more metalcompounds, the metal compounds may be different types of compound, forinstance the substantially crystalline particles may comprise an oxideof one metal and a carbonate of a different metal; e.g. Li₂O and Na₂CO₃.In some embodiments, the substantially crystalline particles maycomprise multiple compounds of the same type but be compounds ofdifferent metals; for example, Li₂O and Na₂O, or Li₂CO₃ and Na₂CO₃. Insome embodiments, the substantially crystalline particles may comprise amixture of metal compounds comprising the same metal, for example, amixture of an oxide and a carbonate of the same metal, e.g. Li₂O andLi₂CO₃.

Some specific metal compounds which may be included in the substantiallycrystalline particles of the inorganic particle mixture include Li₂O,Li₂CO₃, Bi₂O₃, Bi₅O(OH)(NO₃)₄, Ce₂O₃, CeO₂, Na₂O, Na₂CO₃, TeO₂, Bi₂O₃,Ce₂O₃, SiO₂, ZnO, MoO₃, Cr₂O₃ and WO₃.

In some embodiments, any or all of the one or more metal compounds maycomprise multiple metal atoms or ions. For example, in some embodiments,the one or more metal compounds may comprise a compound having thegeneral formula A_(x)B_(y)O_(z) where A is metal and where B is a metaldifferent to A, where 0<x≤2, y is an integer and z is an integer.

Selection of the one or metal compounds of the substantially crystallineparticles may be guided by the desired flow behaviour on firing. Theinventors have found certain mixtures particularly suitable. Forexample, the one or more metal compounds may include a source of alkalimetal, preferably lithium (for example, LiCO₃ or Li₂O) and a source ofbismuth (for example, Bi₂O₃ or Bi₅O(OH)(NO₃)₄).

When the inorganic particle mixture contains substantially crystallineparticles of more than one metal compound, the metal compounds may bemixed (for example, by co-milling) prior to incorporation into aconductive paste and/or prior to mixing with glass frit to form theinorganic particle mixture. In one embodiment, the substantiallycrystalline particles of one or more metals may be milled in combinationwith the glass frit before being incorporated into the conductive paste.

The inorganic particle mixture may comprise ≥0 wt. % to ≤10 wt. %substantially crystalline particles of a tellurium compound (withrespect to total weight of the inorganic particle mixture). In someembodiments, the inorganic particle mixture may comprise 5 wt % or less,3 wt % or less, 1 wt. % or less of substantially crystalline particlesof a tellurium compound. In some embodiments, the tellurium compound maybe tellurium oxide (TeO₂). For example, the inorganic particle mixturemay comprise ≥0 wt. % to ≤10 wt % substantially crystalline particles oftellurium oxide.

The inorganic particle mixture may comprise ≥1 wt. % to ≤9 wt. %substantially crystalline particles of a lithium compound (with respectto total weight of the inorganic particle mixture). In some embodiments,the inorganic particle mixture may comprise at least 1 wt. %, at least 2wt. %, at least 3 wt. % or at least 4 wt. % of substantially crystallineparticles of a lithium compound. In some embodiments, the inorganicparticle mixture may comprise 8 wt % or less, 5 wt % or less ofsubstantially crystalline particles of a lithium compound (Li₂CO₃). Insome embodiments, the lithium compound may be lithium carbonate. Forexample, the inorganic particle mixture may comprise ≥1 wt. % to ≤9 wt.% substantially crystalline particles of lithium carbonate.

The inorganic particle mixture may comprise ≥0 wt % to ≤5 wt %substantially crystalline particles of a sodium compound (with respectto total weight of the inorganic particle mixture). In some embodiments,the inorganic particle mixture may comprise at least 1 wt %, at least 2wt % or at least 3 wt % of substantially crystalline particles of asodium compound. In some embodiments, the inorganic particle mixture maycomprise 4 wt % or less of substantially crystalline particles of asodium compound. In some embodiments, the sodium compound may be sodiumcarbonate (Na₂CO₃). For example, the inorganic particle mixture maycomprise ≥0 wt % to ≤5 wt % substantially crystalline particles ofsodium carbonate.

The inorganic particle mixture may comprise 0 wt. % to ≤5 wt. %substantially crystalline particles of a barium compound (with respectto total weight of the inorganic particle mixture). In some embodiments,the inorganic particle mixture may comprise at least 1 wt. %, at least 2wt. %, at least 3 wt. % or at least 4 wt. % of substantially crystallineparticles of a barium compound. In some embodiments, the inorganicparticle mixture may comprise 4.5 wt % or less of substantiallycrystalline particles of a barium compound. In some embodiments, thebarium compound may be barium carbonate (BaCO₃). For example, theinorganic particle mixture may comprise ≥0 wt % to ≤5 wt % substantiallycrystalline particles of barium carbonate.

The inorganic particle mixture may comprise ≥0 wt % to ≤9 wt %substantially crystalline particles of a cerium compound (with respectto total weight of the inorganic particle mixture). In some embodiments,the inorganic particle mixture may comprise at least 1 wt %, at least 2wt %, at least 4 wt %, or at least 5 wt % of substantially crystallineparticles of a cerium compound. In some embodiments, the inorganicparticle mixture may comprise 8 wt % or less, or 7 wt % or less ofsubstantially crystalline particles of a cerium compound. In someembodiments, the cerium compound may be cerium (Ill) oxide (Ce₂O₃). Forexample, the inorganic particle mixture may comprise ≥0 wt. % to ≤9 wt.% substantially crystalline particles of cerium (Ill) oxide.

The inorganic particle mixture may comprise ≥15 wt % to ≤35 wt %substantially crystalline particles of a bismuth compound (with respectto total weight of the inorganic particle mixture). In some embodiments,the inorganic particle mixture may comprise at least 20 wt %, at least25 wt %, at least 27 wt %, or at least 28 wt % of substantiallycrystalline particles of a bismuth compound. In some embodiments, theinorganic particle mixture may comprise 30 wt % or less of substantiallycrystalline particles of a bismuth compound. In some embodiments, thebismuth compound may be bismuth oxide (Bi₂O₃). For example, theinorganic particle mixture may comprise ≥15 wt % to ≤35 wt %substantially crystalline particles of bismuth oxide.

The inorganic particle mixture may comprise ≥0 wt % to ≤10 wt %substantially crystalline particles of a zinc compound (with respect tototal weight of the inorganic particle mixture). In some embodiments,the zinc compound may be zinc oxide (ZnO). For example, the inorganicparticle mixture may comprise ≥0 wt % to ≤10 wt % substantiallycrystalline particles of zinc oxide.

The inorganic particle mixture may comprise ≥0 wt % to ≤10 wt %substantially crystalline particles of a boron compound (with respect tototal weight of the inorganic particle mixture). In some embodiments,the boron compound may be boron oxide (B₂O₃). For example, the inorganicparticle mixture may comprise ≥0 wt % to ≤10 wt % substantiallycrystalline particles of boron oxide.

The inorganic particle mixture may comprise ≥0 wt % to ≤9 wt %substantially crystalline particles of a tungsten compound (with respectto total weight of the inorganic particle mixture). In some embodiments,the tungsten compound may be tungsten oxide (WO₃). For example, theinorganic particle mixture may comprise 0 wt. % to ≤9 wt % substantiallycrystalline particles of tungsten oxide.

The inorganic particle mixture may comprise ≥0 wt % to ≤9 wt %substantially crystalline particles of a molybdenum compound (withrespect to total weight of the inorganic particle mixture). In someembodiments, the molybdenum compound may be molybdenum oxide (MoO₃). Forexample, the inorganic particle mixture may comprise ≥0 wt % to ≤9 wt %substantially crystalline particles of molybdenum oxide.

The inorganic particle mixture may comprise ≥0 wt % to ≤9 wt %substantially crystalline particles of a zirconium compound (withrespect to total weight of the inorganic particle mixture). In someembodiments, the zirconium compound may be zirconium oxide (Zr₂). Forexample, the inorganic particle mixture may comprise ≥0 wt % to ≤9 wt %substantially crystalline particles of zirconium oxide.

The inorganic particle mixture may comprise ≥0 wt % to ≤9 wt %substantially crystalline particles of a silver compound (with respectto total weight of the inorganic particle mixture). For example, theinorganic particle mixture may comprise ≥0 wt % to ≤9 wt % substantiallycrystalline particles of silver oxide.

In some embodiments, the inorganic particle mixture may comprisesubstantially crystalline particles of any lead compounds in a totalamount of less than 1 wt %, for example, the inorganic particle mixturemay comprise less than 0.5 wt %, less than 0.1 wt %, less than 0.05 wt.%, less than 0.01 wt. % or less than 0.005 wt % substantiallycrystalline particles of any lead compounds (with respect to totalweight of the inorganic particle mixture).

In some embodiments, the inorganic particle mixture may comprisesubstantially crystalline particles of any silicon compounds in a totalamount of less than 2 wt %, for example, the inorganic particle mixturemay comprise less than 1 wt. %, less than 0.5 wt. %, less than 0.1 wt %,less than 0.05 wt %, less than 0.01 wt % or less than 0.005 wt %substantially crystalline particles of any silicon compounds (withrespect to total weight of the inorganic particle mixture).

The particles of the inorganic particle mixture (i.e. the particles ofthe glass frit and the substantially crystalline particles) can bedefined in terms of their D₅₀ and D₉₀ particle size. The terms “D₅₀particle size” and “Do particle size” herein refer to particle sizedistribution. A value for D₅₀ and D₉₀ particle size corresponds to theparticle size value below which 50% and 90%, respectively, by volume, ofthe total particles in a particular sample lie. The D₅₀ and D₉₀ particlesize may be determined using a laser diffraction method (e.g. using aMalvern Mastersizer 2000).

In some embodiments, the inorganic particle mixture has a D₉₀ particlesize of less than or equal to 5 μm, preferably less than or equal to 3μm, more preferably less than or equal to 2 μm. In some embodiments, theinorganic particle mixture has a D₉₀ particle size of at least 1 μm.

In some embodiments, with the caveat that D₉₀ is always bigger than Do,the inorganic particle mixture has a D₅₀ particle size of less than orequal to 2 μm, preferably less than or equal to 1 μm, more preferablyless than or equal to 0.5 m. In some embodiments, the inorganic particlemixture has a D₉₀ particle size of at least 0.2 m.

The particle size distribution of the substantially crystallineparticles of one more metal compounds is preferably similar to that ofthe glass frit. When the crystalline metal compounds of one or moremetals are co-milled in combination with the glass frit to produce theinorganic particle mixture, the milling step can be tuned to produce aninorganic particle mixture with the desired particle size and particlesize distribution. The skilled person will be able to select millingconditions to tune the particle size distribution of the inorganicparticle mixture.

The person skilled in the art will be aware of suitable methods andinstrumentation for calculating particle size distributions. For examplelaser diffraction methods can be used such as use of a MalvernMastersizer 2000.

Organic Vehicle

The solids portion of the conductive paste of the present invention isdispersed in an organic vehicle.

In some embodiments, the conductive paste consists of the solids portionand the organic vehicle.

The organic vehicle typically comprises an organic solvent with one ormore additives dissolved or dispersed therein. As the skilled personwill readily understand, the components of the organic vehicle aretypically chosen to provide suitable consistency and rheology propertiesto permit the conductive paste to be printed onto a semiconductorsubstrate, and to render the paste stable during transport and storage.For example, where it is intended that a paste will be printed by screenprinting or stencil printing, the organic vehicle may be a shearthinning fluid, which fluid may have a high viscosity when at rest butreduced viscosity when subjected to shear stress.

Examples of suitable organic solvents for the organic vehicle includeone or more solvents selected from the group consisting of butyldiglycol, butyldiglycol acetate, terpineol, diethylene glycol dibutylether, tripropyleneglycol monomethylether, Texanol®, dimethyl adipate,2-(2-methoxypropoxy)-1-propanol and mixtures thereof.

Examples of suitable additives include dispersants to assist dispersionof the solids portion in the paste, viscosity/rheology modifiers,thixotropy modifiers, wetting agents, thickeners, stabilisers andsurfactants.

For example, the organic vehicle may comprise one or more additivesselected from the group consisting of Disperbyk 111, Disperbyk 145,Duomeen TDO, fatty acid amides waxes (such as Thixatrol Max, CrayvallacSuper), rosin and its derivatives, acrylic resins (such as Neocryl @),ethyl cellulose, cellulose acetate butyrate, and polyvinylbutyral (suchas Mowital B grade).

The organic vehicle may constitute, for example, at least 2 wt %, atleast 5 wt %, at least 9 wt % of the conductive paste (with respect tototal weight of the conductive paste). The organic vehicle mayconstitute 20 wt % or less, 15 wt % or less, 13 wt % or less, or 10 wt %or less of the conductive paste. In some embodiments, the conductivepaste may comprise 2 to ≤20 wt. %, preferably ≥5 to ≤15 wt. % organicvehicle.

Method of Preparing a Conductive Paste

Typically, the conductive paste is prepared by mixing together theabove-described components of the solids portion and the components ofthe organic vehicle, in any order. In a further preferred aspect, thepresent invention provides a process for preparing a conductive pasteaccording to the first aspect, wherein the process comprises mixingtogether the above-described components of the solids portion and thecomponents of the organic vehicle, in any order.

In some embodiments, the method of preparing the conductive pastecomprises co-milling the substantially crystalline particles of one ormore metal compounds and the glass frit particles of the inorganicparticle mixture before they are mixed with the organic vehicle and theelectrically conductive material.

In some embodiments, the method of preparing the conductive pastecomprises first milling glass frit to produce coarse particles of glassfrit, and then co-milling the coarse glass frit particles withsubstantially crystalline particles of one or more metal compounds toproduce the inorganic particle mixture. The person skilled in the artwill be able to select suitable equipment and milling conditions toproduce the desired particle size and particle size distribution of theinorganic particle mixture.

In some embodiments, the conductive paste of the present invention ispreferably substantially lead-free, that is, any lead-containingcomponents are substantially absent from the paste. For example, theconductive paste may comprise less than 1 wt. % lead.

Manufacture of an Electrode and Solar Cell

The skilled person is familiar with suitable methods for the manufactureof electrodes of a solar cell. Similarly, the skilled person is familiarwith suitable methods for the manufacture of a solar cell. Theconductive paste of the present invention may be employed to prepare abackside electrode or a front side electrode (i.e. light receiving side)of a solar cell. Preferably, the conductive paste of the presentinvention is employed to prepare a front side electrode of a solar cell.

The method for the manufacture of a frontside electrode of a solar celltypically comprises applying a conductive paste onto the surface of asemiconductor substrate, and firing the applied conductive paste.

The conductive paste may be applied to the semiconductor substrate byany suitable method. For example, the conductive paste may be applied byprinting, such as by screen printing, stencil printing or inkjetprinting. In screen printing methods, conductive pastes may be forcedthrough a screen stencil (for example, using a squeegee) onto thesurface of the substrate.

The conductive paste of the present invention may be applied onto asemiconductor substrate to form a front-side electrode of a solar cell.The solar cell may be an n-type or a p-type solar cell. The paste may beapplied onto an n-type emitter (in a p-type solar cell), or onto ap-type emitter (in an n-type solar cell). The paste may be applied ontomono- or multicrystalline semiconductor substrates. The semiconductorsubstrate may be a silicon substrate. Alternative substrates includeCdTe. The surface texture of crystalline substrates may vary dependingon the manufacturing method employed. For example, the surface maycomprise micron-sized, or nano-sized surface features, such as pyramids,inverted pyramids, wells or rods. Such surface features may be formed,for example, by metal catalysed chemical etching (MCCE) or by reactiveion etching (RIE). Examples of such texturized crystalline substratesinclude slurry-cut silicon wafers and diamond wire-cut silicon (DWS)wafers (both also referred to as “black silicon wafer”).

In some embodiments, the solar cell may comprise passivated emitter rearcontact (PERC). Alternative solar cells are known as back junctioncells. In this case, it may be preferred that the conductive paste ofthe present invention is applied to the back-side surface of thesemiconductor substrate of the solar cell. Such a back side surface istypically covered with an insulating passivation layer (e.g. SiN layer),similar to the anti-reflective coating applied to the light receivingsurface of the semiconductor substrate of the solar cell. Alternatively,the conductive paste may be applied to a thin film solar cell or theconductive paste may be applied to a substrate for an electronic deviceother than a solar cell.

The skilled person is aware of suitable techniques for firing theapplied conductive paste. An example firing curve is shown in FIG. 1. Atypical firing process lasts approximately 30 seconds, with the surfaceof the electrode reaching a peak temperature of about 800° C. Typically,the furnace temperature will be higher to achieve this surfacetemperature. For example, the peak surface temperature of the electrodemay be 1200° C. or less, 1100° C. or less, 1000° C. or less, 950° C. orless or 900° C. or less. The peak surface temperature of the electrodemay be at least 600° C.

The semiconductor substrate may comprise an insulating layer on asurface thereof. Typically, the conductive paste of the presentinvention is applied on top of the insulating layer to form theelectrode. Typically, the insulating layer will be non-reflective. Asuitable insulating layer is SiN_(X) (e.g. SiN). Other suitableinsulating layers include Si₃N₄, SiO₂, Al₂O₃ and TiO₂.

Methods for the manufacture of a p-type solar cell typically compriseapplying a back side conductive paste (e.g. comprising aluminium) to asurface of the semiconductor substrate, and firing the back sideconductive paste to form a back side electrode. The back side conductivepaste is typically applied to the opposite face of the semiconductorsubstrate from the front side electrode.

In the manufacture of p-type solar cells, typically, the back sideconductive paste is applied to the back side (non-light receiving side)of the semiconductor substrate and dried on the substrate, after whichthe front side conductive paste is applied to the front side(light-receiving side) of the semiconductor substrate and dried on thesubstrate. Alternatively, the front side paste may be applied first,followed by application of the back side paste. The conductive pastesare typically co-fired (i.e. the substrate having both front- andback-side pastes applied thereto is fired), to form a solar cellcomprising front- and back-side conductive tracks.

The efficiency of the solar cell may be improved by providing apassivation layer on the back side of the substrate. Suitable materialsinclude SiN (e.g. SiN), Si₃N₄, Si₂, Al₂O₃ and TiO₂. Typically, regionsof the passivation layer are locally removed (e.g. by laser ablation) topermit contact between the semiconductor substrate and the back sideconductive track. Alternatively, where pastes of the present inventionare applied to the back side, the paste may act to etch the passivationlayer to enable electrical contact to form between the semiconductorsubstrate and the conductive track.

Examples

The invention will now be illustrated by reference to the followingnon-limiting examples.

Glass Frit Preparation

Glass frits were prepared using commercially available raw materials.The composition of each glass frit is given in Table 1 below. Each glassfrit was made according to the following procedure.

Raw materials for the glass were mixed using a laboratory mixer. Onehundred grams of the mixture was melted in a ceramic crucible, in anelectrical laboratory furnace. The crucible containing the raw materialmixture was placed in the furnace while it was still cold, to avoidthermal shock and cracking of the ceramic crucible. The melting wascarried out at 950-1100° C. in air. The molten glass was quenched inwater to obtain glass frit. The glass frit was dried overnight in aheating chamber at 120° C.

Each glass frit was dry milled in a planetary mill to produce coarseground glass frit having a D₁₀₀ particle size of 200 μm.

TABLE 1 Glass frit compositions Content mol % Frit TeO₂ Li₂O Na₂O BaOCe₂O₃ Bi₂O₃ FRIT A 59.9 16.3 6.3 3.8 3.4 10.3 FRIT B 90.9  9.1 — — — —FRIT C 95.0 — — 5.0 — — FRIT D 95.5 — — — 4.5 —

Preparation of Inorganic Blends

Inorganic blend (i) was prepared by co-milling crystalline particles ofTeO₂, Li₂CO₃, Na₂CO₃, BaCO₃, Ce₂O₃ and Bi₂O₃ in the quantities requiredto provide an inorganic blend having an overall elemental compositionmatching that of Frit A. In the case of Li₂CO₃, Na₂CO₃, BaCO₃ thequantities were such to supply an equivalent molar quantity of the metalhad the oxide of that metal been supplied at the recited mol %.

Inorganic blends (ii) to (v) were prepared by wet milling Frits A to D,respectively.

Inorganic blends (vi) to (viii), (each an inorganic particle mixture asrequired by the paste of the present invention), were prepared byco-milling particles of a glass frit (as prepared above) with therequired crystalline components (i.e. compounds of metals not alreadypresent in the frit composition) in appropriate quantities to provide aninorganic blend having an overall chemical composition matching that ofinorganic particle Frit A. Where non-oxide crystalline components wereemployed, the quantities were such to supply an equivalent molarquantity of the metal had the oxide of that metal been supplied at therecited mol %.

The composition of each inorganic blend in mol % is shown in Table 2.

TABLE 2 Composition of each inorganic blend Overall Inorganic Blendcomposition Inorganic (mol %) Blend Components TeO₂ Li₂O Na₂O BaO Ce₂O₃Bi₂O₃ (i) Crystalline TeO₂, Li₂CO₃, Na₂CO₃, 59.9 16.3 6.3 3.8 3.4 10.3BaCO₃, Ce₂CO₃ and Bi₂O₃ (ii) Frit A 59.9 16.3 6.3 3.8 3.4 10.3 (iii)Frit B 90.9  9.1 — — — — (iv) Frit C 95.0 — — 5.0 — — (v) Frit D 95.5 —— —  4.51 — (vi) Frit B + crystalline Na₂CO₃, 59.9 16.3 6.3 3.8 3.4 10.3BaCO₃, Ce₂O₃ and Bi₂O₃ (vii) Frit C + crystalline Li₂CO₃, 59.9 16.3 6.33.8 3.4 10.3 Na₂CO₃, Ce₂O₃ and Bi₂O₃ (viii) Frit D + crystalline Li₂CO₃,59.9 16.3 6.3 3.8 3.4 10.3 Na₂CO₃, BaCO₃ and Bi₂O₃

For all inorganic blends, the wet milling was carried out in a planetarymill to provide a homogeneous inorganic blend having a D₅₀ particle sizeless than 1 μm (determined using a laser diffraction method using aMalvern Mastersizer 2000). Wet milling was carried out in Dowanol DPMsolvent.

Crystalline TeO₂, Li₂CO₃, Na₂CO₃, BaCO₃, Ce₂O₃ and Bi₂O₃ raw materialsin particulate form were obtained commercially.

Paste Preparation

Conductive pastes each comprising a conductive metal, an inorganic blendprepared as described above and an organic vehicle were prepared inaccordance with the following general method.

88 wt % of a mixture of two commercial silver powders, 2 wt % inorganicblend and 10 wt % of standard organic vehicle were pre-mixed and theresulting mixture was passed through a triple roll mill until ahomogeneous paste was formed.

The mixture of two commercially available silver powders consisted of 4wt % of powder A and 96 wt. % of powder B. Both powder A and Powder Bcomprise a hydrophobic coating. Powder A has an average particle size ofless than 0.8 μm and powder B has an average particle size of less than1.9 μm. Powder A and powder B both have tap density greater than 5 g/mL.

Preparation of Solar Cells

Multicrystalline wafers with sheet resistance of 90 Ohm/sq and being 6inch² in size, were screen printed on their back side with commerciallyavailable aluminum paste and dried in an infra-red mass belt. Eachmulticrystalline wafer was screen printed with a front side conductivesilver paste prepared as described above.

The screens used for the front side pastes had finger openings of 34 μm.After printing the front side, the cells were dried in the infra-redmass belt dryer and fired in a Despatch belt furnace. The Despatchfurnace had six firing zones with upper and lower heaters. The firstthree zones were held at around 500° C., the fourth and fifth zone areat a higher temperature and the sixth zone held at a maximum temperatureof 975° C. (furnace temperature). The furnace belt speed was 610 cm/min.An example firing profile is shown in FIG. 1. The recorded temperaturewas determined by measuring the temperature at the surface of the solarcell during the firing process using a thermocouple.

Testing of Solar Cells Prepared from Conductive Pastes

Fill factor indicates the performance of the solar cell relative to atheoretical ideal (0 resistance) system. The fill factor correlates withthe contact resistance—the lower the contact resistance the higher thefill factor will be. But if the inorganic particle mixture of theconductive paste is too aggressive it could damage the pn junction ofthe semiconductor. In this case the contact resistance would be low butdue to the damage of the pn junction (recombination effects and lowershunt resistance) a lower fill factor would occur. A high fill factortherefore indicates that there is a low contact resistance betweensilicon wafer and the conductive track, and that firing of the paste onthe semiconductor has not negatively affected the pn junction of thesemiconductor (i.e. the shunt resistance is high).

Eta represents the efficiency of the solar cell, comparing solar energyin to electrical energy out. Small changes in efficiency can be veryvaluable in commercial solar cells.

The series resistance represents the sum of the electrical resistancesof particular components of the solar cell. Increases in seriesresistance may result in a direct decrease in fill factor.

After firing the wafers were cooled. After cooling the fired solar cellswere tested in an I-V curve tracer from Halm GmbH, model cetis PV-CTL1.The results are shown in Table 3 below. The results shown in Table 3 areprovided by the I-V curve tracer, either by direct measurement orcalculation using its internal software.

To minimize the influence of the contact area each cell was printedusing the same screen and the same viscosity paste in each individualtest set; this ensured that the line width printed onto each wafer wassubstantially identical and had no influence on the measurementspresented herein.

TABLE 3 Solar cells testing results Inorganic Fill Factor Eta SeriesResistance Example Blend (rel %) (rel %) (rel %) CE1 (i) 100.0 100.0100.0 CE2 (ii) 101.0 99.9 100.1 CE3 (iii) not measurable 21.5 3511.5 CE4(iv) not measurable 10.5 7237.6 Ex. 1 (vi) 100.6 100.7 88.8 Ex. 2 (vii)100.7 101.1 87.0 Ex. 3 (viii)  99.6 100.2 98.6

The examples show that when pastes according to the present inventionare used to prepare solar cells, the resulting solar cells have higherefficiency and lower series resistance as compared to solar cellsprepared from conductive pastes with inorganic blends comprising onlycrystalline metal compounds or only glass frits. Furthermore, theexamples demonstrate that when conductive pastes were prepared using aninorganic blend comprising only Frit B or Frit C that not only could thefill factor not be measured, but very low cell efficiencies and veryhigh series resistances were obtained.

Most notably, whilst the inorganic blends used to prepare the solarcells of CE1, CE2 and Examples 1-3 all have the same overall chemicalcomposition, the solar cells of Examples 1 to 3 have lower seriesresistance and higher cell efficiency.

1. A conductive paste for forming a conductive track or coating on a substrate, the paste comprising a solids portion dispersed in an organic vehicle, the solids portion comprising electrically conductive material and an inorganic particle mixture; wherein the inorganic particle mixture comprises particles of glass frit and substantially crystalline particles of one or more metal compounds; wherein the glass frit comprises greater than 90 mol % TeO₂ and one or more of an alkali metal oxide, an alkaline earth metal oxide, an oxide of cerium, and an oxide of bismuth, wherein the substantially crystalline particles of one or more metal compounds comprise one or more of a lithium compound, a sodium compound, a potassium compound, a barium compound, a cerium compound and a bismuth compound, and wherein the conductive paste is lead-free.
 2. A conductive paste according to claim 1 wherein the glass frit comprises at least 91 mol % TeO₂, preferably at least 92 mol % TeO₂, more preferably at least 95 mol % TeO₂.
 3. (canceled)
 4. (canceled)
 5. A conductive paste according to claim 1 wherein the glass frit is substantially free of silicon oxide.
 6. A conductive paste according to claim 1 wherein the inorganic particle mixture comprises particles of glass frit in an amount of at least 25 wt. %, at least 40 wt. %, at least 45 wt. %, or at least 50 wt. %.
 7. A conductive paste according to claim 1 wherein the inorganic particle mixture comprises particles of glass frit in an amount of 75 wt. % or less, 70 wt. % or less, 65 wt. % or less, or 60 wt. % or less.
 8. (canceled)
 9. A conductive paste according to claim 1 wherein the substantially crystalline particles of one or more metal compounds comprise one or more compounds selected from metal oxides, metal carbonates, metal nitrates, metal hydrogen carbonates, metal oxalates, metal acetates, metal and/formates.
 10. A conductive paste according to claim 1 wherein the substantially crystalline particles of one or more metal compounds comprise one or more of Li₂CO₃, Na₂CO₃, BaCO₃, Ce₂O₃ and Bi₂O₃.
 11. A conductive paste according to claim 1 wherein the inorganic particle mixture comprises substantially crystalline particles in a total amount of at least 20 wt. %, at least 25 wt. %, at least 30 wt. %, at least 35 wt. %, or at least 40 wt. %.
 12. A conductive paste according to claim 1 wherein the inorganic particle mixture comprises substantially crystalline particles in a total amount of 75 wt. % or less, 60 wt. % or less, 55 wt. % or less, or 50 wt. % or less.
 13. A process for producing a conductive paste according to claim 1 comprising mixing an organic vehicle, an electrically conductive material and the components of the inorganic particle mixture, in any order.
 14. A process according to claim 13 comprising the step of co-milling the components of the inorganic particle mixture before they are mixed with the organic vehicle and the electrically conductive material.
 15. A process for producing a solar cell comprising applying a conductive paste according to claim 1 to a semiconductor substrate, and firing the applied conductive paste.
 16. (canceled)
 17. (canceled)
 18. Use of a conductive paste according to claim 1 in the manufacture of an electrode of a solar cell.
 19. (canceled) 